To add a server to a Kafka cluster, it only needs to be given a unique broker id and Kafka must be started on that server. However, until a new topic is created, a new server will not be given any of the data partitions. As a result, when a new machine is introduced to the cluster, some existing data must be migrated to these new machines. To relocate some partitions to the new broker, we use the partition reassignment tool. Kafka will make the new server a follower of the partition it is migrating to, allowing it to replicate the data on that partition completely. When all of the data has been duplicated, the new server can join the ISR, and one of the current replicas will erase the data it has for that partition.